Silicon-containing materials, such as, for example, monolithic ceramics, alloys, intermetallics, and composites thereof have desirable properties for use in structures designed for service at high temperatures in such applications as aeronautical and industrial gas turbine engines, heat exchangers, and internal combustion engines. Environmental barrier coatings (EBCs) are applied to silicon-containing materials to protect the materials from harmful exposure to chemical environments and excessive thermal loads. Thus, EBCs are designed to be thermochemically stable in high-temperature, water vapor-containing environments and minimize interconnected porosity and vertical cracks which provide exposure paths between the material surface and the environment.
EBCs can be single-layer or multi-layer systems, with each layer serving at least one function, such as to provide a thermal barrier, mitigate substrate oxidation or volatilization, or prevent chemical reaction with adjacent layers or the substrate. In many EBC systems, at least one layer is substantially formed from a RE silicate, where the RE includes one or more of the elements Yttrium (Y), Ytterbium (Yb), Holmium (Ho), Thulium (Tm), and Lutetium (Lu). The RE silicates can be, for example, RE monosilicates (RE2SiO5) and RE disilicates (RE2Si2O7). RE silicates have relatively low rates of silica volatilization in combustion atmospheres, low thermal conductivity and excellent thermomechanical compatibility with the above mentioned silicon-containing substrates.
EBC materials can be deposited onto components using a range of coating processes, including thermal spraying (e.g., combustion or plasma spraying), slurry-based deposition (e.g., slurry spraying, dipping, electrophoretic deposition), chemical vapor deposition and physical vapor deposition.
During fabrication or application of the EBC, a large amount of the RE containing silicates is lost as manufacturing scrap. For example, for thermally sprayed EBC layers, as much as 90% of the feedstock powder can be lost to overspray, non-sticking particles, or deposited on tooling fixtures. Collection of this scrap typically results in co-mingling of the high-value, RE-containing material with other, low-value constituents or contaminants. These can include elemental silicon (Si) and silicates of Barium (Ba), Strontium (Sr), and Aluminum (Al) (e.g., such as found in barium-strontium-aluminosilicate (BSAS)).
A similar problem arises with end-of-use EBC-coated components. The coatings can be stripped from these components by processes such as abrasive grit blasting, resulting in a comingled mixture of the abrasive medium, RE silicates, silicates containing Ba/Sr/Al, elemental Si, silicon dioxide, and other impurities resulting from engine operation or the stripping process.
Currently, REs cannot be efficiently recovered from EBC overspray powders, EBC slurry deposition processing, and stripped EBC coatings.